In-fiber integrated optics is an attempt to use silica fiber as a substrate, integrating various optical paths or optical components into a single fiber, to build a functional optical device or component, and to realize a micro optical system, achieving various functions. In-fiber integrated optics is expected to be a new branch of photonics integration. This integration technique enables convenient light beams control and manipulation inside in one fiber. It also provides a research platform with micro and nano scale for interaction between light wave and microfluidic materials. In this review, we briefly summarize the main ideas and key technologies of the in-fiber integrated optics by series integration examples.

Tapered fibers with diameters ranging from 1 to 4 μm are widely used to excite the whispering-gallery (WG) modes of microcavities. Typically, the transmission spectrum of a WG cavity coupled to a waveguide around a resonance assumes a Lorentzian dip morphology due to resonant absorption of the light within the cavity. In this paper, we demonstrate that the transmission spectra of a WG cavity coupled with an ultrathin fiber (500–700 nm) may exhibit both Lorentzian dips and peaks, depending on the gap between the fiber and the microcavity. By considering the large scattering loss of off-resonant light from the fiber within the coupling region, this phenomenon can be attributed to partially resonant light bypassing the lossy scattering region via WG modes, allowing it to be coupled both to and from the cavity, then manifesting as Lorentzian peaks within the transmission spectra. This implies the system could be implemented within a bandpass filter framework.

We theoretically design and experimentally realize a broadband ultrasmall microcavity for sensing a varying number of microparticles whose diameter is 2 μm in a freely suspended microfiber. The performance of the microcavity is predicted by the theory of one-dimensional photonic crystals and verified by the numerical simulation of finite-difference time domain and the experimental characterization of reflection and transmission spectra. A penetrating length into the reflectors as small as about four periods is demonstrated in the numerical simulation, giving rise to an ultrasmall effective mode volume that can increase the sensitivity and spatial resolution of sensing. Moreover, a reflection band as large as 150 nm from the reflectors of the microcavity has been realized in silica optical microfiber in the experiment, which highly expands the wavelength range of sensing. Our proposed microcavity integrated into a freely suspended optical fiber offers a convenient and stable method for long-distance sensing of microparticles without the need for complicated coupling systems and is free from the influence of substrates.

Understanding bend loss in single-ring hollow-core photonic crystal fibers (PCFs) is becoming of increasing importance as the fibers enter practical applications. While purely numerical approaches are useful, there is a need for a simpler analytical formalism that provides physical insight and can be directly used in the design of PCFs with low bend loss. We show theoretically and experimentally that a wavelength-dependent critical bend radius exists below which the bend loss reaches a maximum, and that this can be calculated from the structural parameters of a fiber using a simple analytical formula. This allows straightforward design of single-ring PCFs that are bend-insensitive for specified ranges of bend radius and wavelength. It also can be used to derive an expression for the bend radius that yields optimal higher-order mode suppression for a given fiber structure.

We propose a broadband fiber optic parametric amplifier (FOPA) based on a near-zero ultra-flat dispersion profile with a single zero-dispersion wavelength (ZDW) by using a selective liquid infiltration technique. The amplifier gain and bandwidth is investigated for a variety of fiber lengths, pump power, and operating wavelengths. It is observed that sufficient peak gains and broader bandwidths can be achieved with a small negative anomalous dispersion (β2≤0) and a positive value of the 4th-order dispersion parameter (+ β4) around the pump. We can optimize an FOPA with a bandwidth of more than 220 nm around the communications wavelength.

An ultrasensitive magnetic field sensor based on a compact in-fiber Mach–Zehnder interferometer (MZI) created in twin-core fiber (TCF) is proposed, and its performance is experimentally demonstrated. A section of TCF was spliced between two sections of standard single-mode fibers, and then a microchannel was drilled throughone core of the TCF by means of femtosecond laser micromachining. The TCF with one microchannel was then immersed in a water-based Fe3O4 magnetic fluid (MF), forming a direct component of the light propagation path, and then sealed in a capillary tube, achieving a magnetic sensing element, which merges the advantages of an MZI with an MF. Experiments were conducted to investigate the magnetic response of the proposed sensor. The developed magnetic field sensor exhibits a linear response within a measurement range from 5 to 9.5 mT and an ultrahigh sensitivity of 20.8 nm/mT, which, to our best knowledge, is 2 orders of magnitude greater than other previously reported magnetic sensors. The proposed sensor is expected to offer significant potential for detecting weak magnetic fields.